Human aggression and antisocial behavior are known to be the product of both social and biological risk factors. What is not yet understood is how environment and genetic factors may mediate the interrelationships among these risk factors and antisocial outcomes. A study of twins and their families would provide the ideal opportunity to answer the critical question in this regard: Do measured social and biological variables relate to antisocial development for environment or genetic reasons? Our ability to develop effective and efficient interventions for antisocial behavior rest heavily upon the answer to this question. A study of normal variation in antisocial and aggressive behavior is proposed for 600 twin pairs (both male and female), aged 9-10 years old during an initial assessment, and aged 11-12 years old during a follow-up assessment. The study will provide the first opportunity to investigate the environmental and genetic underpinnings of important social and biological risk factors for unlawful, antisocial and aggressive behavior in boys and girls on the risk of adolescence. Social risk factors will include aspects of the family environment, such as socio-economic status, emotional climate, cohesion, parental warmth and affection, parental supervision, discipline and control. Specific environmental factors for each twin will also be studied including individual relationships with each family member, as well as peer-group characteristics. Biological risk factors will include psychophysiological indicators of arousal (both electrodermal and cardiac channels), as well as neuropsychological and cognitive testing. Both the social and biological risk factors studied here have previously been shown to distinguish between children at high and low risk for aggressive and antisocial behavior. The twin design allows the unique occasion to estimate the relative contributions of environmental and genetic factors to both antisocial behavior and their risk factors, as well as their interrelationships. Of particular importance are whether the links between antisocial behavior with biological and social risk factors, if they exist in this sample, are mediated by genetic or environmental factors. DNA samples will be extracted through mouthwash procedures and will serve two purposes. First, zygosity determination will be made using PCR tests for the 400 same-sex pairs. Then, the remaining DNA will be stored for future analyses of specific genetic loci hypothesized to show associations with antisocial behaviors. We anticipate that by the end of this project, a substantial number of candidate genes will be suggested for antisocial outcomes. At that point, additional funding will be sought to confirm these associations using the rich data source resulting from this project. Detailed knowledge of the underlying biological and genetic mechanisms in aggressive and other forms of antisocial behavior will lead to a greater understanding of how social variables in turn are related to antisocial outcomes, ultimately enabling greater control of this important social phenomenon.